ABSTRACT
The creep nature of metallic spur gear results in the deficiency because of the deformation of
teeth when pressure angle of 20 acting on it. At the replacing points of tooth between driving and driven
the disturbances such as in-evitable random noise, elastic deformation and manufacturing error,
alignment error in assembly all these together causes the high level of gear vibration and noise and
leads to loss in efficiency. The main motto is to reduce the deformation of teeth, by replacing the metallic
cast iron gear with Nylon gear and proved that the deformation of Nylon gear is less compared to
metallic and polycarbonate. Since the deformation is less the loss in efficiency is also less compared to
metallic gear.

KEYWORDS: Gear, Spur gear, nylon spur gear.

I.

INTRODUCTION

The spur gear is simplest type of gear manufactured and is generally used for transmission of rotary
motion between parallel shafts. The spur gear is the first choice option for gears except when high speeds,
loads, and ratios direct towards other options. Other gear types may also be preferred to provide more silent
low-vibration operation. A single spur gear is generally selected to have a ratio range of between 1:1 and 1:6
with a pitch line velocity up to 25 m/s. The spur gear has an operating efficiency of 98-99%. The pinion is
made from a harder material than the wheel. A gear pair should be selected to have the highest number of teeth
consistent with a suitable safety margin in strength and wear. The minimum number of teeth on a gear with a
normal pressure angle of 20 degrees is 18.Mild steel is a poor material for gears as as it has poor resistance to
surface loading. The carbon content for unhardened gears is generally 0.4%(min) with 0.55%(min) carbon for
the pinions. Dissimilar materials should be used for the meshing gears - this particularly applies to alloy
steels. Alloy steels have superior fatigue properties compared to carbon steels for comparable strengths. For
extremely high gear loading case hardened steels are used the surface hardening method employed should be
such to provide sufficient case depth for the final grinding process used.
SPUR GEAR STRENGTH AND DURABILITY CALCULATIONS:
Designing spur gears is normally done in accordance with standards the two most popular series are
listed under standards below:
Bending :The basic bending stress for gear teeth is obtained by using the Lewis formula σ = Ft / ( ba. m. Y )
where F t = Tangential force on tooth, σ = Tooth Bending stress (MPa)b a = Face width (mm),Y = Lewis Form
Factor,m = Module (mm), v Where y = Y/π and p = circular pitch .When a gear wheel is rotating the gear teeth
come into contact with some degree of impact. To allow for this a velocity factor ( Kv ) is introduced into the
equation. This is given by the Barth equation.

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The Lewis formula is thus modified as follows :
Ď&#x192; = K v.Ft / ( ba. m. Y )
Through advancements in plastic resins and manufacturing techniques plastic gears today can be
utilized in a multitude of crucial applications, from transmitting amounts of torque to accurate positioning of
critical components in medical devices. Plastic offers many benefits, including design flexibility and significant
cost savings. Plastic gears possess many advantages over those made of metal. Plastic gears are lighter, quieter,
retain an inherent lubricity and are corrosion resistant. They can be produced in a variety of types,
including bevel gears, offset bevel gears, spiral bevel gears, helical gears, metric gears, metric spur gears, plastic
worm gears and more. Cams, lugs, ribs, webs, shafts and holes can be moulded into plastic gears in one integral
design in a single operation, opening the door to significantly lower production costs.
SPECIFICATION OF EXISTING CAST IRON GEAR:
The typical chemical composition of the cast iron material : Carbon - 2.5 to 3.7%, Silicon - 1.0 to 3.0%,
Manganese - 0.5 to 1.0%, Phosphorus - 0.1 to 0.9% and Sulphur - 0.07 to 0.10%.
SPECIFICATIONS OF NYLON AND POLYCARBONATE PLASTIC MATERIALS:
Chemical composition of Nylon:

Fig 1.1 Chemical Composition Of Nylon
Its properties are determined by the R and R' groups in the monomers. In nylon 6. 6, R' = 6C and R =
4C alkanes, but one also has to include the two carboxyl carbons in the di acid to get the number it donates to
the chain. The majorities of nylons tends to be semi-crystalline and are generally very tough materials with good
thermal and chemical resistance. The different types give a wide range of properties with specific gravity,
melting point and moisture content tending to reduce as the nylon number increases. Nylons can be used in high
temperature environments. Heat stabilized systems allow sustained performance at temperatures up to 185 oC.
Chemical composition of Polycarbonate:
The main polycarbonate material is produced by the reaction of bisphenol A and phosgene COCl2. The overall
reaction can be written as follows:
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Polycarbonates received their name because they are polymers containing carbonate groups (–O–(C=O)–O–).
Most polycarbonates are derived from rigid monomers. A balance of useful features include temperature
resistance and impact resistance.
MATERIAL PROPERTIES OF CAST IRON, NYLON AND POLYCARBONATE:

Since the deflections are less the efficiency of nylon spur gear is more than the cast iron spur gear,
results in less noise and long life, The metallic gear results is more deflection compared to nylon and
polycarbonate, the cost price and life of nylon is also good. When we replace the metallic spur gear with nylon
gear there would be better results we can find in the automobile, robotic and in medical fields where the need of
nylon gear is there.